Heres a list of drives that are known to work in the H1xx and H3xx players.
Avoid drives advertised for ipods and/or ipod branded drives as these normally wont work in a iriver player.
Example of a ipod drive
Note the apple logo on the lower left corner.

Formatting
The drives must be formatted to a FAT32 file system to work in the iriver players and as windows doesn't do FAT32's over 32GB a third party program will be necessary.
Two of my favorite free programs for windows are Fat32Formatter and SwissKnife
Both are user friendly.